1. Field of the Invention
This invention relates generally to guide rolls suitable for guiding hot cast strand in continuous casting, and, more particularly, to a ceramic guide roll construction which can be advantageously used for hot workpieces in continuous casting and other operations.
2. Description of the Prior Art
In continuous steel casting processes, there is an increasing trend toward the energy conservation and simplification of the production line equipment, effectively utilizing the heat of a cast strand in the subsequent hot rolling stage by retaining the heat of the cast strand as it is transferred out of the casing machine after cutting it at a higher temperature. In this instance, the guide rolls which are located before the cutter are required to support a hot cast product of a temperature above 1000.degree. C. as well as a load of a substantial amount.
The conventional guide rolls of this sort are generally constituted by a water-cooled steel shaft of a unitary structure for ensuring the necessary strength, and, in a case where a higher heat resistance is required, such are provided with a surfacing of a heat resistant metal or a ceramic sleeve which is integrally fitted on the steel shaft body. However, the conventional guide rolls of such integral constructions are more or less unsatisfactory in terms of heat resistance, resistance to thermal shocks, resistance to abrasive wear and the strength which is required of the guide rolls to be used for conveying hot cast strand. Consequently, the guide rolls are short in service life and often require replacement, lowering the production efficiency of the casting machine.
With regard to the above-mentioned problems, there has been developed a ceramic guide roll of a composite construction wherein a multitude of discrete ceramic segments are cylindrically assembled around a steel core shaft instead of the conventional integral ceramic sleeve, with a view towards improving the heat resistance properties of the ceramic surfaces and reducing the thermal effects on the steel core shaft by the heat insulating properties of the ceramic material from the standpoint of securing a satisfactory resistance to load of the steel core shaft in addition to advantages in structural strength.
However, a guide roll with a ceramic sleeve consisting of an assembly of discrete segments still has a problem arising from the difference in thermal expansivity between the ceramic sleeve and the steel core shaft. More specifically, except for a guide roll which is divided into a number of sections in the axial direction, the assembled ceramic sleeve is normally greater than the cast steel strip in width, but the temperature of the ceramic sleeve which has a low heat conductivity is elevated to a high level by heat accumulation due to the heat transfer from the cast strip, so that the temperature of the steel core shaft is increased to a considerably high level, for example, to a level of 700.degree. C. at the interface with the ceramic sleeve and to a level of about 400.degree. C. on average.
Under these temperature conditions, since steel has an about 3 to 3.5 times greater thermal expansivity than the ceramic material, the sleeve mounting length which is determined by the segment retainers on the steel core shaft becomes larger than the length of the ceramic sleeve as a result of elongation by thermal expansion of the steel core shaft, the difference in length occurring to a greater degree with a longer core shaft. Consequently, gaps are formed between the ceramic segments, accelerating the heat transmission from the cast strand to the steel core shaft by exposure to the radiant heat of the latter and thus to the thermal fatigue which shortens the service life of the steel core shaft.